Computational modulation is usefully employed in both wireless and wireline applications to implement non-constant envelope modulation schemes such as OFDM or QAM, whereby the term “computational modulation” refers to digitally generated modulation performed by computational means (e.g. by a DSP). For example, a wireline application may be an xDSL transport system and a wireless (RF) application may be the 802.11a wireless LAN standard, or its variants, or Broadband Fixed Wireless Systems such as LMDS or MMDS. In such applications the data is typically scrambled, encoded, and interleaved before being modulated. In the case of wireless applications, the computational modulation is performed before the signal is fed to a digital-to-analog converter (DAC) and subsequently up-converted and amplified for wireless transmission.
OFDM and other related multi-carrier modulation schemes are based on repetitively assigning a multiple of symbols to a multiple of carrier frequencies and calculating the IFFT to obtain the sequential segments of the time waveform to be transmitted. In order to establish phase references for the demodulation decision, training tones are periodically spaced throughout the multiple of carrier frequencies. A significant problem for OFDM modulation is the very high peak-to-average power ratio that may occur during the time sequence output for each IFFT operation. A peak will occur when a majority of the individual carrier frequencies line up in-phase (if a peak appears, it is unlikely that a second one will occur within the same IFFT time segment due to the relatively small number of time samples).
The up-converter and power amplifier of an RF transmitter must perform the frequency shifting and amplification, respectively, of the modulated carrier with a minimum of distortion. In order to achieve a minimum of distortion in a multi-carrier OFDM or single carrier QAM modulation scheme, the up-converter must have a very high dynamic range (i.e. they must be linear and, hence, must have a high compression point). Also, a large power back-off (e.g. 12 dB) for the power amplifier is required due to the high peak-to-average power ratios encountered. Both the high dynamic range requirement and the large power back-off requirement result in a very high DC power consumption for the transmitter and this creates a disadvantage of both OFDM and QAM for wireless or wireline applications.
The designs now being used for the 802.11a 5 GHz wireless standard integrate the transmitter functions of scrambling, encoding, IFFT (Inverse Fast Fourier Transform) generation, modulating, up-converting, and power amplifying without directly addressing the problem of the high peak-to-average power ratio associated with OFDM modulation. However, co-pending U.S. continuation-in-part application Ser. No. 10/205,743 of the assignee of this application, filed on 26 Jul., 2002, provides a signal fragmentation engine which both complements the computational modulation circuitry used in 802.11a architectures and addresses the need for circuitry enabling the use of power efficient, dynamic-range limited RF circuits such as Class D power amplifiers (also referred to as Class S) or Class F Switch Mode power amplifiers and low compression-point up-converters. The contents of co-pending U.S. continuation-in-part application Ser. No. 10/205,743 is incorporated herein by reference.
The reduced peak-to-average power ratios of the fragment signals produced by the assignee's foregoing fragmentation engine are, however, produced at the expense of an associated increase in phase modulation rate (i.e. bandwidth). Therefore, there is a need for an improved fragmentation engine which addresses the increased bandwidth requirements of the foregoing fragmentation engine.